Of the image data that are displayed by a general-purpose display device, there are raster images that are represented by a 2-dimensional array of pixels, and vector images that have information of line. Vector images are represented, for, example, by starting points and ending points of line segments, a combination of the degree of functions and points that define a curved line, or data that specifies an arc, etc.
A raster image maintains the color and brightness for each pixel as data, so that there is no need to calculate them when displaying an image; however, when enlarging the display of the image, the image is enlarged in the unit of pixel, so that roughness of the image stands out. On the other hand, vector image data (vector data) is expanded in a 2-dimensional pixel array in accordance with the resolution of the display, so that when enlarging or reducing an image, it is possible to display the image at that resolution. For example, there is an outline font that maintains the character type face (called a font) in the form of vector data, so that a smooth display can be obtained.
A raster image maintains the color and brightness of all pixels as data, so even when filling in a figure, it can be displayed as is. On the other hand, for a vector image, an area that is filled with color is represented by a closed shape, and only the fill-in color and brightness are specified by data, so that when displaying a filled in area, it is necessary to set the pixels that are included in that area, and to set the pixels in that area to the specified color and brightness. When there are overlapping shapes, the points of intersection and overlapping areas are calculated from the vector data, and the fill-in pixels are set.
In a vector image drawing device that generates contours, an outline buffer is used for storing contour information; however, when it is desired to reduced the capacity of the outline buffer, this is often accomplished by reducing the number of bits in the unit of pixel. Technology for improving improper color values of an image when encoding and decoding image data is disclosed in Patent Literature 1. In the encoding and decoding device disclosed in Patent Literature 1, an encoding unit extracts the difference between an original image and average image, and creates difference data, then performs vector quantization of the difference data to create vector quantization code. Then, the encoding device attempts decoding from the vector quantization code and average image to the original image, and checks whether overflow occurs, and depending on the result, creates exception judgment information. A decoding unit creates difference data from the vector quantization code, and together with decoding the image by adding the difference data and average image, performs exception processing according to the exception judgment information that was created by the encoding processing device when decoding the image.
Patent Literature 2, which is related to technology for improving improper color values in an image, discloses a method of preventing white out in an image due to CCD overflow that occurs because of reflected light in a copier. The technology in Patent Literature 2 set in advance the white level and black level when a CCD read a document, and of the output from a 10-bit A/D converter that digitizes the CCD output, up to the set value of the white level is handled as an 8-bit video signal, and the upper two bits are an overflow detection signal. When overflow is detected, the data of that object pixel is corrected by an image correction circuit using surrounding image data.